A standard widely used in industry for transmission of measurement signals of a physical and/or chemical, measured variable is an electrical current loop. In such case, the measured value of the physical and/or chemical, measured variable corresponds to an electrical current set in the current loop between 4 milliampere (mA) and 20 mA.
Due to drifting and inaccuracies, a somewhat larger electrical current range is permitted, for example, an electrical current between 3.8 mA and 20.5 mA. Electrical currents smaller than 3.6 mA, or greater than 21 mA should not be interpreted by evaluating units as a variable corresponding to the measured value. An electrical current set in the current loop smaller than 3.6 mA or greater than 21 mA is, consequently, identified as error current.
Field devices, especially measuring devices, which have an electrical current requirement of less than 3.6 mA, can be supplied with energy from the current loop. These so-called two-conductor measuring devices (2 L measuring devices) must be connected via a line with only two conductors. Via the two-conductor current loop (2L current loop), the field device is connected with an electrical current source and/or with a process control station. Field devices with a higher electrical current consumption must be supplied with additional, auxiliary energy.
Often it is required, besides the analog electrical signal, to transfer yet other information, for example, calibration-, parameter data, operating temperature and -voltage, etc. via the 2L current loop. In order to transfer this information, a higher frequency signal is superimposed on the measurement signal. A protocol for the transmission of such information is, for example, the HART (Highway Addressable Remote Transducer) protocol.
The analog measurement signal current set in the 2L current loop is, most often, controlled by a control loop in the 2L current loop. In such case, at least one part of the measuring- and operating electronics part is the control loop. For purposes of control, the primary feedback signal of the measuring- and operating electronics corresponding to the actual value of the measurement signal current is read back and compared with the reference input corresponding to the desired value. Usually, the reference input is output from a control unit. The control unit issues the reference input corresponding to measurement signals from a measuring transducer or sensor. In the case of a measured value change, the set actual value in the 2L current loop differs from the desired value. From the control error signal between the actual value and the desired value, the corresponding control signal is produced, by means of which the measurement signal current in the 2L current loop is changed.
In the case of many applications, the size of the transmitted measurement signal can have considerable influence on the surroundings of the measuring device and/or the environment. Especially, in the case of chemical and environment endangering processes, in the case of which a measured value of temperature, to fill level, pressure, flow or composition of a measured substance is transmitted, a safe running of the process is decisive. Therefore, it is extremely important, that the transmitted measurement signal actually correspond to the measured value. Devices and systems in safety-relevant applications must, consequently, satisfy special requirements, among other things, fulfill the standards for functional safety (e.g. iEC 61506, IEC 61511 etc.). Central component of the standards for functional safety is the so-called safety integrity level (SIL). Depending on the danger emanating from a process or a plant, a corresponding SIL-capability is required for an application. The SIL specifies, with which probability an arising defect of a device or an application is detected. In such case, one distinguishes between a SIL of 1 to 4, wherein 1 is the lowest and 4 the highest safety level.
From Offenlegungsschrift EP 1 860 513 a circuit is known, which assures, that an electrical current set in a current loop corresponds to the actual value of the measured variable to be represented. The circuit relies on the redundant setting of the value of the loop current and a monitoring of the same. While redundantly designed, electrical, electronic and/or electronically programmable components used in the state of the art do, in given cases, increase the functional safety of equipment, there is, nevertheless the disadvantage of increased manufacturing costs for the redundant design of the system and the components.
Furthermore, known from U.S. Pat. No. 7,098,798 B2 is a circuit for monitoring an electrical current in a 2L current loop. For such purpose, the voltage drop across a resistor is measured by means of an evaluating circuit. A disadvantage is that the resistor is not monitored and a malfunction stemming from the resistor is not recognized.